Method of manufacturing low carbon high tensile strength alloy steel



United States Patent METHOD OF MANUFACTURING LOW CARBON HIGH TENSILESTRENGTH ALLOY STEEL Joseph R. Zanetti, Dearborn, Mich., assignor toNational Steel Corporation, a corporation of Delaware No Drawing. FiledAug. 11, 1967, Ser. No. 659,885 Int. Cl. 021d 7/00 U.S. Cl. 148-12 2Claims ABSTRACT OF THE DISCLOSURE A low carbon, weldable, high tensilestrength alloy steel characterized by ductility and toughness suitablefor structural uses is produced in the as rolled condition by utilizinga steel consisting essentially, by weight, of

Percent, about and the balance iron with residual impurities. The steelis hot rolled to form bars, plate and sheets with a starting rollingtemperature between 2100 F.-2300 F. and a finishing rolling temperaturebetween 1400 F.1700 F. followed by air cooling.

This invention is concerned with low carbon, high tensile strength,alloy steel. More specifically it is concerned with low carbon, hightensile strength, alloy steel having a minimum tensile strength of about75,000 p.s.i. in the as rolled condition.

Low carbon, high tensile strength steels of the prior art have requiredheating, quenching, and tempering treatment subsequent to rolling inorder to develop the desired tensile strengths and suitable ductilityand toughness. Such treatments require multiple and uneconomicalhandlings, e.g., the product must be held rigidly to prevent warpingwhile being cooled rapidly from a temperature about 1750 F., and higher,to room temperature in about 10 to 25 seconds.

The quenching develops 100% marstenitic steel and is required to obtaindesired high tensile strength. Without such quench treatment, suchsteels have a tensile strength of about 50,000 to 60,000 p.s.i. in theas rolled condition.

After quenching, such steels previously required a tempering treatmentat about 850 F. to about 1300 F. to develop ductility and otherproperties necessary for end product usage.

This invention provides an alloy steel with high tensile strengths inthe range of about 80,000 to about 120,- 000 p.s.i. (minimum tensilestrength of 75,000 p.s.i.). In addition, the steel exhibits ductilityand yield point elonice gation values suitable for normal fabrication ofhigh tensile strength steels and toughness suitable to withstandingnormal stresses encountered in use of such steels. Obtaining theseresults Without treatment subsequent to rolling is one of the highlyunexpected contributions of the invention.

The low carbon high tensile strength alloy steel of the presentinvention consists essentially of the following weight-percentages:

Percent, about Carbon .07-.20 Manganese .75-1.15 Silicon .50-1.0

Chromium .50-.80 Zirconium .05-.20

Boron .0005-.0025 Titanium .15 max. Columbium .01-.06

the balance being iron with residual impurities such as are ordinarilyencountered in conventional basic oxygen, open hearth, or electricfurnace practice used in producing the steel. The maximum desirablevalues of the most common residual impurities encountered are:

Phosphorus .04 max. Sulphur .05 max. Copper .35 max. Aluminum .07 max.

The upper limit of carbon is ordinarily not exceeded in order tomaintain the good weldability of end product provided by the invention.Carbon content below the lower limit is ordinarily avoided in order tohave sufiicient carbon available for providing desired tensile strength.

The manganese is added in the furnace or the ladle. Where possible,e.g., in open hearth practice, it is added in both the furance and theladle. Preferably, the manganese is maintained in the range of .85% to1.0% with a specific value of .90% being desired. The manganese helpsincrease the yield point and tensile strength of the steel and aids coldformability.

Silicon acts as a deoxidizer and ferrite strengthener. It can be addedin the furnace or ladle; where possible silicon is added in both.Silicon is desirably kept in the range of .75 to with .85% beingpreferred.

The chromium is added in the ladle and acts to increase both the yieldpoint and tensile strength of the steel. A preferred specific value forthe chromium is .65%.

The zirconium acts to increase hardenability, refine grain structure,increase toughness and cold formability. It is added in the ladle and ispreferably kept between .075% and .12%; with .08% being the specificvalue aimed for.

The boron is added in the ladle. It increases hardenability of the steeland seeks out remants of nitrogen in the steel to a greater extent thanother nitride formers such as chromium, zirconium, columbium. Thetitanium helps to protect the boron for this purpose because of its 3greater affinity for oxygen than boron. A preferred value for the boronis .0010% and a preferred range is .0005 to .012%.

The titanium also acts as a carbide former and is preferably kept atabout .03%. The titanium should be added with the boron.

Columbium contributes to a number of the unexpected 4 these propertiesare set forth in the representative examples below:

EXAMPLE No. 1

Low carbon columbium treated alloy steel bars having a minimum yieldstrength of 75,000 psi. Dimensions: 5%" x .250 x 100"; 4 /2" x .312 x110".

M11 P S Si Cr Zr B Tl Cb Ladle chemistry 17 94 013 010 90 74 08 0012 03038 Recheek chemistry:

.250 ga .17 .94 .87 .75 .09 .0014 .03 .035 .312 ga .17 .97 .88 .76 .10.0014 .03 .037

MECHANICAL PROPERTIES Elong. Bars YS (p.s.i.) TS (p.s.i.) percent, in.Bend As rolled 2-4-8 110, 620 29-21-15 SB 2 /Xt 106, 930 29-21-16 SB %Xt111, 360 24-16-11 SB 56x2 103,760 107, 180 28-17-14 SB 105, 140 30-22-16SB 1 0.2% ofisct. 2 Short break.

CHARPY V IMPACT TEST DATA [Ft.-lb., at temperature F.]

Rm Thickness Direction temp 32 0 20 -40 -60 .250" Transverse 12. 7 9.06. 7 4. 8 3.0 2. 2 .394 x .197 x 2.165 Longitudinal. 24. 3 17. 7 13. 813. 8 6.0 4. 2 .312 18. 7 10. 7 6. 5 4. 5 2. 8 2. 7 .394" x .295 X 2.165Longitudinal 39. 0 18.0 12. 3 6.0 3. 8 3. 2

EXAMPLE NO. 2

0 Mn P S Si Cu Cr Zr Al B Ti 01) Ladle chemistry- 17 92 010 024 78 03 7006 041 0007 03 Reeheck chemistry:

.250 ga 18 90 009 025 79 04 70 08 041 0007 03 050 .3125 ga 17 87 009 02578 04 68 07 040 0007 03 055 1 Added in mold.

MECHANICAL PROPERTIES Brinnell Hardness Elong., Bars No. YS (p.s.i.) TS(p.s.i.) in. TS/YP Bend As rolled- 2-8 .250 ga 81, 600 119, 730 14-21 1.37 1 t 0.2% offset.

CHARPY V IMPACT TEST DATA [Ft-lb. at temperature F.]

Thickness KIT 32 0 -20 .250 (.394X.197 2.165) 7. 0/4. 7 5. 5/3. 3 4.7/3. 0 3. 2/2. 5 2. 3/2. 0 (l/t) results obtained through the invention;It raises the yield point significantly without detrimental efiect onthe highly desirable ductility and toughness obtained by the inventionin the as rolled condition. The columbium is added in the ladle, or canbe added in the mold. A preferred specific value for the columbium is.035 and the preferred range is .035% to .06%.

If the steel is degassed during processing the ladle additions are madeafter degassing to minimize oxidation products and reduce possiblecontamination of the steel.

The alloy steel of this invention is rolled into bars, plate or sheetswhich are suitable for structural uses, railway car framing, and thelike. This steel is characterized by high tensile strength, goodweldability, high energy absorption capacity, and good ductility andyield point elongation values in the as rolled condition. Data on Theprocess taught by the present invention, in addition to or incombination with the chemistry, also contributes to the desirablecharacteristics obtained. In practice the alloy steel is provided inbillet form, that is in a suitable form for rolling into bars, plates orsheets. The billets are heat soaked at about 2100 to 2300 F., preferablyabout 2150 F., prior to rolling. The billets are then rolled. Multiplestand rolling is carried out so that finishing rolling temperature ofthe product of the present invention can be about 200 to 300 lower thanthe finishing rolling temperatures employed with high tensile strengthsteels in the prior art. The finishing rolling temperature should be inthe range of 1400 to about 1700 F.

After rolling the product is allowed to cool without quenching or forcedcooling. Such cooling will ordinarily take place at a rate of about toper second at least through the major portion of the cooling dependingon the shape and thickness of the final product.

Other advantages of the invention are inherent in the above description.It is understood that changes and modifications in the foregoingteachings can be made without departing from the spirit of the inventionso that the scope of the invention is to be determined from the appendedclaims.

What is claimed is:

1. Process for producing low carbon high tensile strength alloy steelbar and plate suitable for welding and characterized by minimum tensilestrength of 75,000 p.s.i. in the as rolled condition without subsequentheat treatment comprising the steps of providing an alloy steel billetconsisting essentially, by

weight, of

Percent, about Carbon 0.7-.20 Manganese .75-1.15 Silicon .50-1.0Chromium .50-.80 Zirconium .05-20 Boron .0005.0025 Titanium .15 max.Columbium .01-.06

and the balance iron with residual impurities,

heat soaking such alloy steel billet at a temperature between about 2100F. and about 2300 F., hot rolling the steel billet to desireddimensions, with a hot rolling starting temperature being between about2100 F. and about 2300 F. and a hot rolling finishing temperaturebetween about 1400 F. and about 1700 F., and allowing the hot rolledsteel to cool without quenching or forced cooling. 2. The process ofclaim 1 in which the hot rolling starting temperature is about 2150 F.and the hot rolled steel air cools at a rate of about 5-10 per second.

References Cited UNITED STATES PATENTS 3,0l0,822. 11/1961 Altenberger148-12 3,102,831 9/1963 Tisdale 148-12 OTHER REFERENCES Making, Shapingand Treating of Steel, by US. Steel, 7th edition, pp. 386-387.

HYLAND BIZOT, Primary Examiner US. Cl. X.R. --126 mg UNITED STATESPATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,544,393 DatedDecember 1, 1970 Inventor(a) Joseph R. zgnetti It is certified thaterror appears in the above-identified paten and that said Letters Patentare hereby corrected as shown below:

. r- Column 2, line 39, "furance" should read furnace (p. 4, line 3)Column ,2, line 47, "90%" should read .9o%

' (page 4, line 11) Column 2, line 57, "remants" should read remnants(page 4, line 20) Colurrin 5, line 18, carbon percent should read .O7.20% (claim 4, line 8) Signed and sealed this 28th day of September1971.

(SEAL) Attest:

EDWARD M.FLETCHER,. IR. ROBERT GOTTSCHALK Attesting Officer ActingCommissioner of Paten

